Tree mixers, also known as analog multipliers or Gilbert multipliers, have been widely used in integrated circuits for communications systems for many years. As is known for example from A. Bilotti, "Applications Of A Monolithic Analog Multiplier", IEEE Journal of Solid-State Circuits, December 1968, pages 373 to 380, such a mixer or multiplier typically comprises a first or lower differential pair of common emitter transistors to the bases of which a first differential analog input signal is supplied, and two second or upper differential pairs of transistors whose bases are supplied with a second differential analog input signal and whose collector-emitter paths conduct the currents of the lower pair of transistors to produce in their collector circuits an analog output signal which represents the product of the input signals. A single current source in the emitter circuit of the lower pair of transistors provides bias current to all six transistors. For use as a mixer in a radio communications receiver or transmitter, for example an input signal is applied to the lower pair of transistors and a local oscillator signal is applied to the two upper pairs, or upper quad, of transistors.
Such a circuit provides advantages of good rejection of the input signals at the output, good power supply rejection, and the possibility of conversion gain. It has a disadvantage in that them is a trade-off between noise and distortion, which limits the achievable dynamic range. Noise can be reduced, but distortion is increased, by decreasing the bias current and resistance in the emitter circuit of the lower pair of transistors. Conversely distortion can be reduced (linearity increased) by increasing these parameters, but this increases noise, especially shot noise from the upper quad of transistors, this being proportional to the bias current.
In an attempt to improve this trade-off, P. E. Chadwick, "The SL6440 High Performance Integrated Circuit Mixer", Plessey Semiconductors Limited, 1981, discloses that complementary linearising transistors can be added to the lower pair of transistors to provide negative feedback. However, this approach has the disadvantages of introducing noise and presenting bias stability problems for practical ranges of operating temperature and integrated circuit process parameters.
In Yamasaki et al. U.S. Pat. No. 5,182,477, issued Jan. 26, 1993 and entitled "Bipolar Tunable Transconductance Element", there is described a transconductance element which can be used as a biquad filter and in which another degree of freedom is added to the determination of pole frequency and pole quality factor by incorporating additional current sources. Although this patent refers to a Gilbert multiplier, the circuit described is in fact a Gilbert gain cell with only one input for an a.c. signal, and an input for a d.c. control voltage. The circuit of this patent is not a mixer producing an output which represents the product of two input signals, and provides no frequency conversion as in a tree mixer. Although this patent also refers to implementing dual-input or multiple input transconductance elements, this is clearly seen from the patent to refer to a sum, not a product, of the input signals. Thus the operation of the circuit of this patent is completely different from the operation of a tree mixer.
An object of this invention is to provide a tree mixer in which the disadvantages discussed above are reduced.